Selective loop antenna system for receivers



June 30, 1970 BOUBQULEIX ETAL 7 3,518,550

SELECTIVE LOOP ANTENNA SYSTEM FOR RECEIVERS Filed June 28, 1956 3 Sheets-Sheet l 1 0 i024 o o W FIG.2 FIG.3

FIG/1' June 30, 1970 BQUBQULEIX ETAL 3,518,550

SELECTIVE LOOP ANTENNA SYSTEM FOR RECEIVERS Filed June 28. 1966 3 Sheets-Sheet 2 I I l l I I o I l g on 1 I -00 I I 1 5 g i m l i 4) I I "U I i q-- E l l l l o 4 7 km "1 q r 2% 23 a June 30, 1970 aouaou l EIAL 3,518,550

SELECTIVE LOOP ANTENNA SYSTEM FOR RECEIVERS Filed June 28, 1966 3 Sheets-Sheet 5 l i' I l l l l I l l United States Patent 3,518,550 SELECTIVE LOOP ANTENNA SYSTEM FOR RECEIVERS Albert Boubonleix and Ren Cheze, Paris, France, as-

signors to CSF-Compagnie Generale de Telegraphie Sans Fil, a corporation of France Filed June 28, 1966, Ser. No. 561,097 Claims priority, appliczation France, July 2, 1965,

Int. Cl. H0315 3 /04; H04b 1/16 US. Cl. 325-375 6 Claims ABSTRACT OF THE DISCLOSURE This present invention relates to receiver antenna systerns.

More particularly, it is an object of the invention to provide a high frequency receiver antenna system which is both compact and selective.

According to the invention there is provided an arrangement for picking up radioelectric waves comprising a loop antenna; a plurality of ferrite rods parallel to the axis of said loop and peripherally mounted therein; a preamplifier having an input coupled to said loop and positioned in the immediate vicinity thereof, said preamplifier comprising at least one amplifier stage including a negative feed back loop and a crystal circuit, resonant at the operating frequency, said crystal circuit being mounted in parallel with said feedback loop.

For a better understanding of the invention and to show how the same may be carried into effect, reference will be made to the drawings accompanying the following description and in which:

FIGS. 1 and 2 show respectively a lateral and a first view of a loop aerial according to the invention, partly in section;

FIG. 3 shows a detail of the loop aerial;

FIG. 4 is a graph for choosing the optimum dimensions of the loop aerial; and

FIG. 5 is the electrical diagram of a loop unit and of a preamplifier unit according to the invention.

The aerial shown in FIGS. 1, 2 and 3 comprises:

A loop formed of a number of circular turns, for example four (11 to 14), which can be connected in series or in parallel by means of a selection block 2 for each sub-band;

Ferrite rods 31 to 39 parallel to the axis of the turns which are located inside the loop to increase the equivalent area of the latter; the length/diameter ratio of these rods is about to ensure at the centre a permeability coefiicient, no, close to the permeability of the toroid;

An electrostatic screen 4 of generally cylindrical shape, slit along a generatrix 40 and surrounding a preamplifier unit 5 incorporated in the aerial;

A cable 6 connected to preamplifier 5 by a coaxial connector 57;

An adjustable support 8 consisting of a cylindrical insulated ring 81, in which are mounted turns 11 to 14 and the ferrite rods 31 to 39, and an adjustable base 82 rotationally mounted on a fixed base 83 through which passes the aerial cable 6 protected by a tube 84;

Patented June 30, 1970 ice A glass-fibre envelope 9, formed of two hemispherical shells 91 and 92, which is fixed against the adjustable support ring 81 through sealing gaskets 93, 94; the two hemispherical shells are assembled together and secured to ring 81 by assembly screws 95, 96 which pass through the aerial at right-angles to ring 81.

FIG. 3 shows, on a larger scale, the preamplifier envelope 5 and indicates the location of the removable elements provided for choosing the sub-band: quartz crystal 51 and jumper 52 for inserting, between the crystal and ground, a resistance for matching the crystal.

FIG. 4 shows the method used for determining the dimensions to be given to the above-described loop to ensure that the quality of the system, characterised by the signal-to-noise ratio, shall be satisfactory.

One has:

M Signal Nu P Noise Na+Nr+Np Na+Nr where:

Nu is the level of the useful signal Na is the aerial noise level Nr is the receiver noise level Np is the level of the interfering signal Signal 02 Noise 1 FIG. 4 shows as ordinates the signal-to-noise ratio expressed in decibels with respect to its maximum value a, and as abscissa the parameter ,8.

This curve has a knee at around {3:2, which corresponds to a signal-to-noise ratio of 1.75 db. When ,8 increases above 2, the signal-to-noise ratio increases much less rapidly than for its increase up to 2.

It is therefore advantageous to choose the loop dimensions such that 18:2.

Hence Signal g M g Noise 3 Np 3 and Np=2FbNa Np corresponds to the level of atmospheric interference transmitted by the aerial to the receiver input and depends only on the aerials characteristics, for a given interference field.

Writing P for the power supplied by the aerial in the presence of noise only:

P is a known function of the loops form factor Ff, of the received frequency f, of the loops off-load Q-factor Q, of the loops diameter d and of the field H:

and

f is expressed in cycles per second d is expressed in meters H is expressed in volts per meter.

For determining the dimensions of the antenna, one assumes Ff is the ratio of the loops diameter to the loops length expressed in the same units, that P=Np and that it has the minimum value of the parasitic field; one then selects the value of the coefficients Ff and Q.

In the term ZFbNa, the receiver noise figure Fb is assumed to be given and aerial noise is proportional to the absolute temperature and to the pass-band B:

Na=K.T.B., where K is Boltzmanns constant T is the absolute temperature,

the aerial being assumed to be matched to the receiver input impedance.

When, by the above method, the diameter d of the loop of the aerial has been determined, there are two cases to be considered:

(a) The parameter of the loop is small compared to the wavelength corresponding to the frequency used.

For example, for a working frequency of 30 ,u.C./S., a loop with a diameter not greater than 30 cm. gives a diameter wavelength ratio of 5.3, which reduces the aerials effective height by a little more than 1%, which may be taken to be acceptable.

(b) The calculated diameter exceeds the acceptable value, which would involve an excessive reduction of the effective height at the top of the frequency band.

In the latter case, the calculated diameter of the loop will have to be reduced and a suflicient number of ferrite rods will be used to increase the equivalent area of the loop, while keeping the diameter down to an acceptable value.

The power gain corresponding to the addition of n rods of cross-section Sb is equal to the square of the ratio of the useful area Su to the area Sc of the loop:

where ac. is the permeability coefficient of the rods, its value being close to the permeability of the toroid.

Knowing the desired ratio Su/Sc, it is possible to determine the number n of rods required for sufficiently limiting the aerial effect.

The circuit diagram of FIG. corresponds to a loop aerial incorporating a preamplifier according to the invention.

The block 2 comprises six connection boxes 21 to 26 making it possible to adapt the loops symmetrical circuit to six sub-bands. Each box includes a capacitor, 210 to 260, of fixed value corresponding to one sub-band inserted between two capacitors 110 and 140 whose values are adjustable to suit the chosen sub-band frequency. The corresponding loop connections are shown in the FIG. 5, the light upper plots of each box, between which the connections are inserted, respectively corresponding to the eight terminals of the loops 11 to 14.

The preamplifier shown in FIG. 5 consists of three silicon transistor stages 53, 53a and 53b which provide an overall amplification of 26 db, with a noise figure of 6 db. In addition to its amplifier function, amplifier 5 comprises a quartz crystal tuned to the frequency of the signal to be received and behaves as a selector.

The piezoelectric crystal 51 is inserted in the emitter of a first transistor53 in a common emitter circuit. Jumper 52 provides a choice of resistances 521 to 525 in order to adjust the pass-band to suit the frequency.

The current in transistor 53 is determined by a transistor 54, in a grounded base circuit, whose emitter collector space transmits an emitter voltage to this transistor 53 and whose dynamic output impedance, in parallel with 4 the impedance of crystal 51, is very high. This allows the first stage 53 to be excited without substantially damping the high-Q circuit of the crystal.

The terminals '58 and 59 are at the same time the HF output of the antenna and a DC input for supplying the transistors stages.

For the resonance frequency of the quartz crystal, i.e. for the frequency of the signal to be received, the crystal behaves as a circuit tuned in series, i.e. as a short-circuit, and the negative feedback is low, Accordingly, the gain of the amplifier is highfOutside the desired band, the impedance of the crystal is high and so is the negative feed back. Accordingly the amplifier gain is low.

In order to compensate for the parallel capacity 51a (shown in dotted 11115) of cr stal 51 Whose effect might become dominant outside the crystals pass-band, a part of the high-frequencycurrent at the stage input is applied through an adjustable capacitor 56 to the emitter of transistor 54-. It is collected from thecollectorof transistor 54, after a phase shift of and applied, at 55, ahead of the crystal 51, so compensating for-the high-frequency current in the parallel capacitor 51a.

This compensation ensures a high negative feed-back for frequencies outside the resonance frequency, whereas for the crystals resonance frequency the crystal offers a low impedance corresponding to a very low negative feedback factor. 1

This ensures high selectivity in the preamplifier.

Preamplifier 5 thus fulfills three functions:

(a) It amplifies the signal, and atmospheric noise, before the aerial downlead and soreduces the relative eifect of noise collected in the downlead;

(b) It improves the noise figure of the combined aerial and receiver;

(c) It is provided with a crystal filter which, Without adversely affecting the noise figure, adds a highly selective filtering action to that already obtained by virtue of the loops high Q-factor.

Of course, the invention is not limited to the embodiments shown and described which were given solely by way of examples.

What is claimed is:

1. An arrangement for picking up radioelectric waves comprising a loop antenna, a plurality of ferrite rods parallel to the axis of said loop and peripherally mounted therein, the total section area of said ferrite rods being much smaller than the area delimited by said loop antenna; a preamplifier having an input coupled to said loop antenna and positioned in the immediate vicinity thereof, said preamplifier comprising at least one amplifier stage having a plurality of electrodes, one of said electrodes being coupled to a point having a fixed potential through a resonant circuit forming a negative feedback, said resonant circuit including a piezoelectric crystal resonating at the operating frequency of said arrangement and means, serially connected with said crystal, for adjusting the damping of said resonant circuit.

2. An arrangement as claimed in claim 1, wherein said loop antenna comprises a plurality of identical coaxial turns and means for connecting selectively in parallel and in series said turns.

3. An arrangement as claimed in claim v1, wherein said amplifier comprises a first transistor stage having a common emitter circuit, said crystal circuit being inserted in said emitter circuit, a second transistor having a grounded base circuit, an emitter, a collector and supplying D.C. voltage to said first transistor through the space included between said emitter and said collector, said means comprising a plurality of resistors and means for selectively inserting one of said resistors between said crystal circuit and said point.

4. An arrangement as claimed in claim vll, further comprising two hemispherical sheels coaxial with said loop antenna and enclosing said loop antenna and said preamplifier.

5 6 5. An arrangement as claimed in claim 4, wherein said 2,641,561 6/1953 Black 343-872 shells are made of glass fibre. 2,972,145 2/ 1961 Watts 343-742 6. An arrangement as claimed in claim 4 further com- 3,075,191 1/1963 Peay 343-872 prising a support carrying said loop antenna, said shells, said preamplifier and a cylindrical electrostatic screen slit 6 FOREIGN PATENTS along a generatrix and surrounding said loop antenna and 1,046,697 12/1958 y- Said Preamplifier- KATHLEEN CLAFFY, Primary Examiner References Cited 0. JIRAUCH, Assistant Examiner UNITED STATES PATENTS 10 1,547,154 7/1925 Hull 325 -486 1,747,008 2/1930 Jacobson 343868 330-21, 31; 343788, 867 

